1. Technical Field
This invention relates to a periodic fuel injector designed to inject fuel pulses of variable quantity and timing into the cylinder of an internal combustion engine. In particular, this invention relates to an improved means for spilling fuel from an injection chamber of the fuel injector at the end of an injection stroke in order to achieve a sharp end of injection.
2. Background Art
To achieve a sharp end of injection, unit type fuel injectors typically spill the trapped volume of fuel in the injection chamber with a spill port, at the end of an injection stroke. A sharp end of injection is desirable in order to increase engine performance, improve fuel efficiency and abate undesirable exhaust emissions.
During the downward stroke of an injector plunger, extremely high injector pressure must be attained, e.g., 15,000 psi or greater, to insure that a sufficient quantity of fuel can be injected within the short interval during each injector cycle when injection should occur. Unless injection takes places at exactly the right time, engine performance can degrade dramatically. High pressures are also essential to insure that the fuel entering the combustion chamber is adequately atomized and mixed with the compressed air. Fuel pressures as high as 30,000 psi have been found to be desirable in some injector designs.
To achieve such high pressures, the unit injector's plunger must be accelerated to a relatively high velocity during its injection stroke and must be very carefully matched with the central bore of the injector body in which the plunger is designed to reciprocate in order to avoid fuel leakage.
The extremely high fuel pressure at which the injector is required to operate further exacerbates the need for very close tolerance because the high pressure causes the injector body to dilate. Unless the injector body is made rigidly to resist substantial high pressure induced dilation, fuel leakage and unpredictable fuel pressure losses may occur.
The need for fuel injection at high pressure also complicates the need for very accurate injection timing as discussed above. For example, high injection pressure requires high plunger velocity but such high velocities lead to difficulties in achieving a sharp end of injection. In particular, high pressure fuel injection can be terminated by causing the injector plunger to engage a stop but such engagement may cause the plunger to bounce back and thus produce a dribbling effect which can lead to poor combustion, reduced fuel efficiency and increased emissions.
To avoid the problem described above, it has been proposed to provide a slightly raised dimple on the cam lobe controlling the plunger in order to place a "crush load" on the injector plunger at the end of the injection event and thereby hold the plunger very tightly against a stop such as an injector cup. See, e.g., Perr U.S. Pat. No. 4,471,909. While this arrangement provides sharp fuel cut-off, it also places stresses on the plunger actuation mechanism, thus adversely affecting the durability of the fuel injection system.
FIG. 3 depicts a fuel injection chamber spill port arrangement in accordance with Perr et al. U.S. Pat. No. 4,463,901, the entire contents of which is hereby incorporated by reference. FIG. 3 shows lower plunger section 150 in its lowermost position, wherein the volume of the injection chamber is brought to a minimum and a fuel drain passage extension 194, including a radial portion 194a and an axial portion 194b form a path of communication between the injection chamber and fuel drain passage 188 in order to quickly reduce the pressure within the injection chamber 162 to produce a positive and predictable end to the injection event. This also reduces the requirement for a large "hold down" force to be created by fluid in a timing chamber, thus reducing camshaft loading.
In the above prior art arrangement, the small amount of fuel discharged through fuel drain extension 194 and passage 198 is recirculated back to the fuel supply. Final downward movement of the lower injector plunger ceases upon contact of the lower injector plunger with the upper surface of a tip valve spring housing 127. It can also be seen in FIG. 3 that a similar spill port 158 is provided for spilling fuel from a timing chamber defined between upper injection plunger 148 and intermediate plunger section 152.
Walter et al. U.S. Pat. No. 4,235,374 similarly discloses a unit injector provided with spill ports for collapsing a timing chamber and dumping fuel from an injection chamber.
A problem exists with spill ports of the type just mentioned. Spill ports have high leakage when located in injector barrels which are being dilated by high injection pressure. Furthermore, such spill ports can tend to side load the plunger if there are not provided multiple ports to balance the forces, thus creating wear which can result in fuel leakage within the injector assembly. Also, spill ports are typically rectangular EDM'ed ports which are costly and difficult to locate accurately with respect to a spill groove necessarily provided in the plunger.
Salisbury U.S. Pat. No. 1,852,191 discloses a centrally located spill port arrangement to allow termination of injection before the injection plunger completes its working stroke. The spill port is opened by a linkage and actuating mechanism which operates independent of the plunger. Thus, a complicated separate mechanism is required to spill fuel from the injection chamber.